Conventional drilling techniques often employ drilling fluid (termed “mud”) that is circulated downhole for various reasons such as carrying earth cuttings out of the wellbore, cooling the drill bit, and also to control pressure in the well. The mud is pumped downhole through the drillstring, where it exits at the bottom of the drill bit and is forced to the surface in the annular space between the drillstring and the wellbore (hereinafter “annulus”). The hydrostatic pressure exerted by the mud column is the primary method of controlling the pressure in the formation. Loss of pressure and circulation problems may occur due to the mud being lost to the formation rather than circulating back to the surface. Although drilling can continue under these adverse conditions, it is important that lost circulation be detected as early as possible for safety and well control reasons.
First, is that drilling fluid is expensive (e.g. $50-$300 per barrel), and pumping thousands of barrels into the formation drastically increases drilling costs and may cause formation damage. Second, if the circulation downhole is lost, the cuttings are not removed from the wellbore, and surface interpretation of changes in the rock formation cannot be detected. Also if downhole circulation is lost and cuttings are not removed from the hole, the cuttings may “settle” in the wellbore, thereby “sticking the drillstring” in the hole. Third, when the formation pressure exceeds the hydrostatic pressure exerted by the mud column, a “well kick” may occur where formation fluid unexpectedly enters the well. Uncontrolled fluid entry from the formation can lead to a dangerous condition known as a “well blowout.” Thus, a method and apparatus for detecting and monitoring fluid flow downhole at any point along the drillstring would be very desirable.
Presently, technologies such as surface monitoring of the level of mud in the mud pit, or measuring the mud inlets and return lines are employed. Loeppke et al., describes a rolling counterbalanced float flowmeter to be used in the return lines in “Development and Evaluation of a Meter for Measuring Return Line Fluid Flow Rates During Drilling,” Report SAND91-2607, Sandia National Laboratories, Albuquerque, N.Mex. (June, 1992). U.S. Pat. No. 6,257,354 issued on Jul. 10, 2001 to Schrader et al., details a flow velocity sensor for mud return line measurement. However, these surface measuring technologies fail to provide timely response to a well kick deep in the well because of the amount of travel time it takes for the pressure transients in the mud to reach the surface.
U.S. Pat. No. 4,527,425 issued on Jul. 9, 1985 to Stockton (hereinafter '425) discloses a down-hole mud flow rate detector consisting of an acoustic transmit-receive pair positioned on the outer wall of the drillstring to measure return mud flow rate in the annulus and another transmit receive pair on the inner wall of a drill string to measure incoming mud flow rate. Differences in the acoustic transit time between up-stream and down-stream directions along the incoming mud flow and return mud flow are measured and used to determine the averaged flow velocities inside the drill pipe and in the annulus. However, this “transmit-time” method may be subject to several possible problems. First, the pulse wave from the transmitter is non-directional to the receiver and thus may be subject to beam diffraction and acoustic attenuation in the fluids along the path lengths. Second, the received waveform likely has a formation echo train which may consist of fast compression wave, slow compression wave, shear wave, or Stoneley wave that may interfere or overlap with the fluid echo and can make accurate determination of timing of the fluid echo very difficult. Third, invariably there are variations in the speeds of sound in the formations and/or the mud on both the incoming and return paths due to different pressures, temperatures, and unexpected fluid composition due to a well kick. This local variation in the speed of sound may exacerbate the aforementioned problems, thereby making accurate determination of the transmit time difference due to the annular flow even more difficult. Lastly, the transit time method taught in '425 only provides averaged velocity and not the full point-velocity profile across the annular gap.
In summary, conventional techniques do not provide in-situ measurements of the velocity profile of drilling mud within the wellbore or the direction of flow (i.e., target moving towards or receding from the transducers in the axial, radial, and tangential directions in the annulus).